Organic synthetic fibers have been hitherto widely used in clothes and industrial materials because they have excellent fiber properties. However, in the field where heat resistance is required, inorganic fibers such as asbestos, glass and steel are predominantly used and organic synthetic fibers are scarcely utilized.
Nevertheless, recently, development of heat resistant organic synthetic fibers has been conducted earnestly due to linking of the remarkable progress in organic synthetic chemistry with various needs in clothes, industrial materials, aviation and space developments and the like. As a result, various organic synthetic fibers have been developed. Among them, a representative which has achieved extreme success in commercial scale production is meta-wholly aromatic polyamide fibers mainly composed of poly-m-phenyleneisophthalamide (hereinafter abbreviated as PMIA).
PMIA fibers can be used within a working temperature range of 50.degree. to 200.degree. C. higher than that of known synthetic fibers, and they also have general properties necessary for general-purpose fiber products such as, for example, balanced strength and elongation, flexibility, post-processability and the like. Further, because the fibers have such a very high flame retardance with self-extinguishing characteristics that they do not flame up upon combustion and are extinguished immediately after removing flame, the fibers are utilized in various fields such as industrial materials, for example, heat resistant filter mediums, electrical insulating materials, etc.; clothes, for example, anti-heat protecting suits (e.g., fireman's suits, flying clothes, clothes for furnace workers, etc.); bedclothes; and the interior decoration field, and the range of their use is still increasing.
However, it has been found that PMIA fibers are yet insufficient for use in clothes such as anti-heat protecting suits and the like where form stability at a high temperature, for example, higher than the melting point of fibers is required. In order to deal with this point, it has been proposed to admix a small amount of para-wholly aromatic polyamide fibers [Seiji Tata, Plastic 36, 34 (1985)]. In this method, form stability at a high temperature is improved depending upon the mixing ratio. However, there is such a defect that flexibility and post-processability of PMIA fibers which are comparable to those of fibers for general-purpose clothes are drastically impaired because para-wholly aromatic polyamide fibers have extremely high stiffness and extremely low elongation for use as fibers for clothes.
Another problem is that, upon combustion, a product made of PMIA fibers are remarkable deformed due to heat shrinkage with causing firm fusion between fibers thereof to each other, although melt drip by melting of the fibers is not caused. Therefore, when such a product is accidentally burnt up during putting on as an anti-heat protecting suit, it is difficult to take off the suit, which makes an injury such as a burn rather worse.
Further, PMIA fibers are deficient in dyeing properties due to their polymeric construction and therefore they are not suitable for the field of clothes, particularly, for the fashion industry. In order to improve their dyeing properties, introduction of, for example, sulfone group is employed. However, other properties of the fibers are impaired due to such introduction, while improvement of dyeing properties is yet insufficient. In addition, apart from piece-dyeing with dyes, so-called solution dyed fibers colored with pigments are marketed. However, variety of colors is limited and further colors are limited to deep ones.